Method and device for continuously stretch-bend-leveling metal strips

ABSTRACT

The invention relates to a method and a device for continuously stretch-bend-leveling metal strips, wherein a strip under a tensile stress below the elastic limit is alternatingly bent about at least four straightening rollers in the plastic or elastic-plastic region and thereby undergoes plastic stretching. The bending radii of all four straightening rollers are thereby set dually and independently of each other, as the straightening rollers are set up having individually controlled positions.

The invention relates to a method and an apparatus for stretch-bend-leveling metal strips, where a strip under tension below the elastic limit or the yield point is bidirectionally bent in the elastic/plastic range about at least four straightening rollers and thereby undergoes plastic stretching. The plastically or elastically/plastically acting rollers are also designated as stretch rollers. The amount by which the strip is (overall) plastically stretched and therefore elongated is called the stretch ratio.

With such a stretch-bend-leveling method, nonplanar metal strips can be straightened and therefore, nonplanarity can be eliminated. Nonplanarity means, for example, strip waviness and/or strip cambers that are a result of length differences of the strip fibers in the strip plane. However, nonplanarity also means strip curvature in the longitudinal and/or transverse direction that result from bending moments in the strip, for example if the strip was elastically/plastically bent about deflection rollers or is caused by elastic/plastic deformations when winding up the strip. Longitudinal curvatures are also designated as coil set; transverse curvatures are designated as cross bow. During stretch-bend-leveling, the nonplanar strip is bidirectionally bent about rollers with an appropriately small diameter and under a tension that lies below the elastic limit R_(E) or the technical elastic limit R_(p0.01) of the strip material so that with the superimposition of the tension with the bending, an elastic/plastic deformation in the strip is generated. The strip is plastically elongated by an the amount called the stretch ratio. In the case of the plastic elongation, the initially short strip fibers are elongated. In the ideal case, all strip fibers have the same length after straightening so that in theory an ideally straightened strip without waviness or strip camber is produced.

With the stretch-bend-leveling methods known from practice, it is in principle possible that after the straightening, due to the bidirectional bending in the elastic/plastic range, residual bending moments remain in the strip that can become visible as cross bow and, in the case of a cut-out sheet, can result in a plastic residual curvature in the longitudinal and/or transverse direction. The residual bending moments are caused if the individual bends are not optimally adapted to each other in terms of intensity. The bending radii depend on the strip parameters (thickness, modulus of elasticity, cyclic strength behavior, Poisson's ratio), the tension in the strip, the roller diameters and the geometry of the strip track directly engaging the rollers. In a first approximation, the geometry can be described as the wrap angle of the strip about the rollers. In the case of a sufficiently large wrap angle or a sufficient tension, the strip shapes to the radius of the roller. Then, the strip curvature reaches its maximum and remains constant while the wrap angle or the tension continues to increase. However, the wrap angles are usually set in such a manner that the strip, at least in the case of the last rollers in the stretch-bending stand, no longer conform to the roller radius. Even with an optimal setting of a specified stretch-bending stand, residual bending moments occur due to fluctuations of the process parameters. The reason for this is that, in practice, the tension and accordingly the stretch ratio as well as the strength values and the strip thickness are in principle subject to certain fluctuations. The stretch-bend-leveling methods known from practice are to a greater or lesser extent susceptible to such fluctuations. This means that in the case of the conventional stretch-bend-leveling methods, such fluctuations influence the remaining or generated residual curvatures to a greater or lesser extent. This applies also to stretch-bend-leveling methods in which the strip is elastically/plastically deformed about four straightening rollers.

For such stretch-bend-leveling stands having four straightening rollers it is known to set the geometry of the stretch-bend-leveling stand in order to achieve the best possible straightening results. For example, a stretch-bend-leveling stand with four straightening rollers is known from DE 696 08 937 T2 (EP 0 767 014 [U.S. Pat. No. 5,758,534]), where the upper straightening rollers as well as the lower straightening rollers can be changed with respect to their position. While the lower straightening rollers can be positioned with a certain accuracy via mechanical spindle drives for improving the straightening result, the upper straightening rollers are actuated hydraulically. In this manner, a desired overlap of the rollers is adjustable.

A similar configuration applies to the stretch-bend-leveling stands known from DE 695 14 010 T2 (EP 0 665 069 [U.S. Pat. No. 5,666,836]) and DE 38 85 019 T2 (EP 0 298 852 U.S. Pat. No. [4,898,013]).

The professional article “Benefits of a new leveler technology for packaging steels: MultiRoller Tension Leveler” (Emmanuel Dechassey, Irsid, Arcelor group, METEC Congress June 2003) describes in a similar manner a stretch-bend-leveling apparatus with four straightening rollers. The overlap of he first straightening unit (first and second rollers) and the overlap of the second straightening unit (third and fourth rollers) are adjustable.

DE 691 01 995 T2 (EP 0 446 130 [U.S. Pat. No. 5,127,249]) mentions also the possibility of changing the position of all four straightening rollers. However, this serves primarily for changing between a waiting position and a working position. The reason for this is that in the case of a stretch-bend-leveling stand, it is normally necessary to open the stand so that a joint between two coils or strips can pass through, and to subsequently close it again. In any case, practical experience has shown that the known stretch-bend-leveling stands and stretch-bend-leveling methods are relatively susceptible to fluctuations of the process parameters so that residual curvatures cannot be minimized in the desired manner.

In principle less susceptible to the process parameters are so-called multiroll levelers that are also designated as multiroller straightening apparatuses. During multiroller straightening, the strip is subjected to a multiplicity of bends about rollers having a small diameter. By using many rollers, the strip can be produced with minor residual curvatures. A disadvantage here is the fact that the multiplicity of rollers involve high expenditures for maintenance and spare parts. Moreover, the lower and the upper rollers are each frictionally engaged with each other via the support rollers or intermediate rollers and support rollers like a 1:1 gear. Since the strip, when running through the multiroller stand, is plastically elongated, the strip speed increases to the same extent. Thus, the strip runs synchronously only at one roller; between the other rollers and the strip, a slip occurs. The rollers in high-speed trip processing lines run often with high rotational speed. The slip can induce vibrations that can generate chatter marks on the strip surface. In practice, this is not acceptable for a variety of applications. For these reasons, conventional stretch-bend-leveling stands were combined with multiroller straightening units (cf. EP 0 665 069 B1 or DE 695 14 010).

Apart from this, DE 27 50 752 [GB 2,007,556] describes a method for straightening and for improving the quality properties of metal strips, in particular electrical strips, where the metal strip to be treated is bent without counter pressure and is subjected to a plastic alternating bending. While maintaining its plastic bending state, the metal strip is bent in a flat position, where the deflections of the metal strip following the last bending are carried out exclusively in the elastic or near-elastic range. Preferably, only two bending rollers without counter pressure rollers are implemented while positioning one or a plurality of deflection rollers therebetween or downstream thereof. It is also possible to provide a plurality of bending rollers. It is always important that the last bending is carried out with a plastic bending deformation as high as possible and subsequently, the metal strip is only bent elastically or predominantly elastically.

Based on the known prior art, it is an object of the invention to provide a stretch-bend-leveling method that, with low installation-related expenditures, delivers optimal flatness results and is in particular not susceptible to fluctuations of the process parameters (tension, strip strength and strip thickness). In addition, an apparatus shall be proposed that is characterized by a simple and cost-effective construction and yet delivers the desired results.

To achieve this object, the invention teaches for a generic method for continuously stretch-bend-leveling metal strips of the above-described kind that the bending radii are set at all four straightening rollers individually and independently of each other. For this, the invention proposes that all four bending rollers are adjusted in a controlled manner. If the strip is bidirectionally bent about more than four straightening rollers, the invention proposes that the bending radii at least of the last four rollers with regard to the strip travel direction are set individually and independently of each other by preferably adjusting at least the last four rollers individually in a controlled manner.

Studies using a calculation model that considers stress in the longitudinal and transverse directions and, as calculation parameters, considers the strip thickness, the modulus of elasticity, the Poisson's ratio, the cyclic strength behavior of the strip, the tension in the strip, the roller diameters and the geometry of the strip track about the rollers, have surprisingly shown that this method delivers excellent residual curvature results and is in particular relatively unsusceptible to fluctuations of the process parameters. Of particular importance is here that the four elastically/plastically acting bendings can be adjusted or are set independently of each other with a sufficient accuracy and in a position-controlled manner. The positions of the straightening rollers are then set with an accuracy as well as control accuracy that corresponds to an accuracy with regard to the wrap angles of the strip about the straightening rollers of ±0.05° or less, preferably ±0.02° or less. The wrap angles of the strip about one or a plurality of straightening rollers, preferably about all straightening rollers, can be 0.5° to 60°, preferably 1° to 35°. The (total) stretch ratio is, for example, 0.1% to 1.5%, preferably 0.2% to 0.6%. The tension in the strip is preferably approximately 30% to 60% of the elastic limit of the strip or the strip material.

The simulation calculations have shown that a two-roller configuration for stretch-bend leveling usually does not achieve the goal because the longitudinal and transverse bending moment curves as a function of the strip curvatures are not negatively mirror-imaged to each other as is the case for pure bending, but are shifted relative to each other due to tension. Thus, for correcting plastic strip curvatures in the longitudinal and transverse directions generated at the first roller, a single roller is not sufficient because with it only the longitudinal or the transverse bending moments can be balanced. The minimal number of rollers for generating an ideal strip without residual curvatures with given process parameters is therefore three. However, calculations have shown that the achieved residual bending moments or the elimination of such residual bending moments in a stretch-bend-leveling stand with three rollers is relatively susceptible to fluctuations of the process parameters. The same applies to a four-roller stand in which the individual rollers cannot be adjusted individually and independently of each other with the required accuracy. This problem is solved here by the method according to the invention in which the four rollers are adjusted individually and independently of each other in a position-controlled manner.

The invention further proposes that the spacing between at least two straightening rollers provided (directly) one behind the other (with regard to the travel direction of the strip) is at least 15% of the (maximum) strip width, for example at least 30% of the (maximum) strip width, particularly preferred at least 50% of the (maximum) strip width. It can be useful here if these spacings for all four straightening rollers are provided between each pair of two straightening rollers located (directly) one behind the other. For example, the spacings between two such immediately adjacent straightening rollers can be at least 150 mm, preferably at least 300 mm. Model-based studies have led to the surprising result that also through a suitable selection of the spacings between the elastically/plastically acting straightening rollers, the process-related differences of the stretch ratio over the strip width can be minimized or compensated. It was found that in general larger horizontal spacings have a beneficial effect. With a given maximal installation length of the stretch-bend-leveling stand it is useful to implement at least one spacing between elastically/plastically acting rollers with at least 30% of the maximum strip width, but at least with 500 mm. A decisive factor here is the spacing of the straightening rollers along the strip travel direction. In a horizontally designed stretch-bend-leveling stand, this corresponds to the horizontal spacing. However, a spacing in a diagonal or vertical direction can in principle also be meant, depending on the direction in which the strip is guided.

Depending on the spectrum of applications of the products to be straightened, different roller spacings are optimal so that a further improvement of the straightening results with regard to the waviness can be achieved if these spacings are variable or adjustable. Therefore, according to a further proposal of the invention that is independently important, the spacing in the strip travel direction between at least two straightening rollers is set or can be set in a variable manner. The optimal spacings for a certain strip can then be predetermined based on the model.

The invention further proposes that the target values for the position of the straightening rollers and/or the target values for the wrap angles of the strip about the straightening rollers are determined with a calculation model that considers at least stress in the longitudinal and transverse directions and, as calculation parameters, processes the strip thickness, the modulus of elasticity, the Poisson's ratio, the cyclic strength behavior of the strip, the tension in the strip, the roller radii and the geometry of the strip track about the rollers, for which the target values of the strip thickness and the strength, for example the yield point, are calculated such that the residual longitudinal curvature and the residual transverse curvature become zero or at least negligible. Thus, in the ideal case, strips with extremely small residual curvatures are generated. However, there is the optional possibility of determining the target values with the calculation model with the proviso that, on the one hand, an extremely small or negligible residual curvature value is achieved, but also that the residual curvatures take a defined value. The invention is based on the knowledge that there are in fact applications in which residual transverse curvatures have to be avoided completely, in which, however, certain residual longitudinal curvatures can be tolerated as long as they are present exclusively in one defined direction and therefore have only one defined sign.

Optionally, the invention further proposes that downstream of or behind one or a plurality of straightening rollers, for example downstream of the last straightening roller, particularly preferred downstream of each straightening roller, the cross bow of the strip over the strip width is measured. These measured cross bows can then be combined in a controller that, based on the measured values, corrects the wrap angles or roller positions in such a manner that a strip with a sufficiently small residual longitudinal and residual transverse curvature is generated.

Accordingly, the invention comprises in the first instance such embodiments for which the target values for the bending radii and therefore for the roller positions and/or the wrap angles are determined by a mathematical model, where the installation is then operated using these values. Simulations have shown that such approaches, even with certain fluctuations of the process parameters, deliver excellent results. Optionally, there is the possibility of correcting the target values during the startup of the installation or, where applicable, also during the operation, namely on the basis of experimentally obtained results. Such a correction can be carried out “offline.” As described, there is the optional possibility of performing an “online” correction taking into account the measured values as a controller working with or without feedback. This feedback control can also be based on a calculation model that considers at least stress in the longitudinal and transverse directions and that, as calculation parameters, processes the strip thickness, the modulus of elasticity, the Poisson's ratio, the cyclical strength behavior of the strip, the tension in the strip, the roller radii and the geometry of the track of the strip about the rollers, and where a residual longitudinal or transverse curvature is assigned to a certain cross bow.

The described calculation models can be based on a finite element method that is able to consider stress and elongations also over the strip width. Based on the expected maximum strip nonplanarity or the nonplanarity measured before straightening, the plastically-elastically acting rollers' optimal stretch ratio and/or optimal horizontal spacings, which result in minimal residual nonplanarity, here in particular waviness and strip camber, can be calculated using the finite element method.

The invention is also an apparatus for continuously stretch-bend-leveling metal strips with a method of the described kind. Such an apparatus has at least four bending rollers about that a strip under a tension below the elastic limit is bidirectionally bent in the plastic or elastic/plastic range. Such an apparatus further comprises at least one controller working with or without feedback. The invention proposes that the bending radii at all four straightening rollers can be set individually and independently of each other. For this purpose, the invention preferably proposes that all four straightening rollers are connected to the controller and can be adjusted in a controlled manner. For this, the individual straightening rollers are each provided with one or a plurality of finely adjustable actuators, preferably screw jacks. The diameter of one or a plurality or preferably all four straightening rollers is, for example, 15 mm to 150 mm, preferably 25 mm to 80 mm. It has already been explained that particularly preferred relatively large spacings between the individual straightening rollers are used in operation. Particularly preferred, the apparatus is configured such that these spacings are adjustable in that, for example, the position of at least one straightening roller along the strip travel direction can be variably set.

Another requirement for a straightening process, apart from having no residual curvature, is to minimize waviness/strip camber. Waviness present in the strip prior to straightening is to be eliminated and the straightening process itself should not generate new waviness. Stretch-bend-leveling involves the problem that the strip deforms elastically/plastically at the roller and thereby is plastically elongated. The plastic longitudinal elongation is accompanied by a plastic reduction of the strip width. This means the strip section immediately upstream of the roller is wider than the one immediately downstream of the roller. However, since the strip cannot rapidly change width, shear stresses develop in the strip plane that vary along the strip width and the strip length. The shear stresses in turn result in an overall uneven plastic deformation over the strip width, therefore in different plastic longitudinal elongations over the strip width and thus in waviness caused by the straightening process. This waviness is usually center waviness that increases with increasing stretch ratio. The model-based studies have led to the surprising result that these process-related differences of the stretch ratio in the strip width can be minimized or compensated by a suitable selection of the spacings between the elastically/plastically acting rollers. It was found that, in general, larger horizontal spacings have an advantageous effect. Depending on the spectrum of applications of the products to be straightened, different roller spacings are optimal so that a further improvement of the straightening result with regard to the waviness is achieved if at least one straightening roller is horizontally adjustable.

For large horizontal spacings of the elastically/plastically acting straightening rollers, the immersion depths at given wrap angles become relatively large. For passing through a joint between two coils, for example a punch connection or a welding seam, the stretch-bending stand usually has to be opened, that is, the rollers are moved apart. If the spacings traveled in the course of this are long, this process takes a long time. If in the case of a continuous line, the strip continues to run during this time, as a consequence, the unstraightened strip lengths around the joint become large. This is in principle undesirable. As an optional refinement, the invention therefore proposes that upstream and/or downstream of at least one elastically/plastically acting straightening roller, an adjacent deflection roller is provided that preferably has at least three times the diameter, particularly preferred at least ten times the diameter of the straightening roller. In this manner, the positioning spacings and therefore the unstraightened strip sections are significantly shortened. The deflection rollers have such a diameter that for at least a portion of the strip thickness range to be straightened only a purely elastic strip deformation occurs. Preferably, at least the last two rollers should be implemented without intermediate deflection rollers in order to achieve the highest possible accuracy when setting the wrap angle.

Moreover, it is possible that at least one of the adjacent deflection rollers has a concave or convex roller shape. Furthermore, one of the two adjacent deflection rollers or also both adjacent deflection rollers can be equipped with a roller bending mechanism in the horizontal and/or vertical direction. A shape-variable tensioning roller or deflection roller wrapped by at least 120° can be provided upstream of the first elastically/plastically acting straightening roller. With this measure it is achieved that at least at one elastically/plastically acting straightening roller, a certain tension distribution over the strip width is set so as to be able to influence the stretch ratio distribution over the strip width in such a manner that after straightening, a strip with the lowest possible residual waviness is obtained. Specifically, increasing the tension at the strip edges would counteract center waviness.

Apart from that, elastically/plastically acting straightening rollers can be supported against deflection by support rollers. For example, segmented support rollers can be used. For instance, the elastically/plastically acting rollers can be supported against deflection by two rows of segmented support rollers or two intermediate rollers and three rows of segmented support rollers.

In the following, the invention is explained in more detail with reference to a drawing illustrating one embodiment. In the figures:

FIG. 1 is a simplified schematic view of an apparatus according to the invention for stretch-bend-leveling,

FIG. 2 is a view like FIG. 1 showing a modified embodiment,

FIG. 3 is a diagram of calculated residual longitudinal curvatures for a metal strip that has been straightened with an apparatus according to FIG. 1, and

FIG. 4 is a diagram showing the residual longitudinal curvature produced by a conventional three-roller stand with a single adjustment.

The figures illustrate an apparatus for continuously stretch-bend-leveling metal strips 1. In the illustrated embodiments, the apparatus has four straightening rollers 2 about which a strip 1 under a tension below the elastic limit is bidirectionally bent in the plastic or plastic-elastic range. Each of the straightening rollers 2 is supported in a manner known per se by at least two support rollers 3. The strip 1 is under a tension below the elastic limit. For this purpose, unillustrated tensioning rollers are provided, for example a set of brake rollers at the upstream end and a set of draw rollers at the upstream and downstream end.

According to the invention, the bending radii can be set for each of the four straightening rollers case individually or independently of each other. For this, all four straightening rollers 2 can be adjusted in a position-controlled manner by an unillustrated controller and a respective unillustrated actuator, for example screw jacks, namely in a direction V vertical to the strip travel direction R. This is indicated in the figures by the functional positions of the rollers 2 illustrated with dot-dash lines. The parameters and in particular the roller radii and the tension in the strip are adapted to each other in such a manner that the strip at all four straightening rollers does not quite conform to the roller radius. Accordingly, by controlling the adjustment depth and therefore by controlling the position and the resultant setting of the wrap angle, the respective bending radius can be varied.

In the embodiment according to FIG. 1, no further rollers are provided between the four straightening rollers 2. Only upstream and downstream of the entire straightening roller arrangement are there deflection rollers 4.

In contrast, FIG. 2 shows a modified embodiment in which one deflection roller 4 a is provided upstream and one deflection roller 4 b is downstream of each straightening roller 2. In this embodiment too, the positions of all four straightening rollers 2 can be varied in a position-controlled manner perpendicular to the strip travel direction R.

Although the embodiment according to FIG. 1 is in principle preferably used because, except for the elastically acting pass-line rollers at the upstream and downstream end, it does not need any further (purely elastically acting) rollers, the optional embodiment according to FIG. 2 can be of advantage. Combinations of the two embodiments are also possible. In terms of the purely elastically acting deflection rollers it should be noted that they act elastically only in the case of thin and/or high-strength strips, for example aluminum in the strip thickness range below one millimeter. Moreover, they have an elastic/plastic effect, although a minor one, and can influence the straightening result with regard to residual curvature. If a stretch-bend-leveling stand is to cover a wider strip thickness range, it is particularly preferred that the deflection rollers are dispensed with.

For the startup of such an installation, the exact target positions of all four straightening rollers 2 are determined by a mathematical model. When doing this, it is possible to first determine the target wrap angle with a calculation model that considers at least stress in the longitudinal and transverse directions and, as calculation parameters, processes the strip thickness, the modulus of elasticity, the Poisson's ratio, the cyclic strength behavior of the strip, the tension in the strip, the roller radii and the geometry of the strip track about the rollers, for which the target values of the strip thickness and the strength, for example the yield point, are calculated in such a manner that the residual longitudinal curvatures and the residual transverse curvatures become zero or at least almost zero. In this manner, the positions of the straightening rollers 2 can be exactly set. This opens the possibility, depending on the experimental results, to perform “offline” a correction of these positions.

Particularly important is the fact that the initially specified setting ensures that the residual curvature behavior is extremely resistant to fluctuations of the process parameters such as, in particular, the tension and the strength values and the strip thickness of the strip. Since in practice, a strip of a coil is usually subject to certain strength and thickness fluctuations and, moreover, the tension cannot always be exactly maintained, the residual curvatures obtained with the previously known methods were also subjected to significant fluctuations. This is avoided within the context of the invention. The method according to the invention shows a particularly gentle residual curvature behavior. In this connection, reference is made to a comparison of FIGS. 3 and 4.

FIG. 3 shows the residual longitudinal curvature k-L (in 1/m) as a function of the stretch ratio S (in %) for an aluminum strip that was straightened with an apparatus according to FIG. 1. These are calculated values generated with a mathematical model. In the illustrated embodiment, a yield point of 250 MPa with fluctuations of ±10 MPa and a strip thickness of 0.28 mm with fluctuations of ±0.05 mm were assumed. The limit values of the residual longitudinal curvature of ±0.5 m⁻¹ considered as being permissible are also shown in the figure. It is apparent that the residual longitudinal curvature in the specified tolerance window lies within a rather large stretch-ratio range of 0.31 to 0.59%. Moreover, the fact that the residual longitudinal curvature fluctuates in a stretch ratio window of ±0.05% only by 0.15 m⁻¹ is remarkable. The residual transverse curvatures not shown here are always smaller than the residual longitudinal curvatures.

Furthermore, the residual longitudinal curvatures within a stretch ratio window of 0.36 to 0.52% are limited to a negative range with very little fluctuations. By adapting the wrap angle at the last roller, the residual curvature profile in the diagram can be shifted upward or downward. In this manner, a defined positive residual longitudinal curvature can be set.

In comparison, FIG. 4 shows a corresponding simulation with a three-roller stand, where the individual rollers are likewise adjustable in a position-controlled manner. It is apparent that in the case of fluctuations of the stretch ratio, the residual longitudinal curvature fluctuates in a significantly wider range. The limit value is met only in a range for the stretch ratio of 0.33 to 0.36%. In practice, for example in acceleration and deceleration phases, this can hardly be allowed for.

Apart from that, it is overall remarkable that the described results are achieved with relatively few straightening rollers so that overall the installation is characterized by a simple and cost-effective construction.

If a variant is operated with more than four straightening rollers, at least the last four straightening rollers can be individually set according to the invention with regard to their wrap angles so that a gentle residual curvature behavior is achieved. When selecting suitable wrap angles with the method according to the invention, the absolute value of the residual transverse curvature is significantly lower than the one of the residual longitudinal curvature. Thus the residual longitudinal curvature of a plate cut out after straightening corresponds approximately to the transverse curvature of the strip measured in the stretch-bend-leveling machine with the strip under tension. It is therefore optionally proposed to measure the cross bow at least at the downstream end, and from the measured value to conclude the residual longitudinal curvature and in the case of a deviation from the desired value, to change the setting of the wrap angle and therefore also the position until the cross bow corresponding to the desired residual longitudinal curvature is obtained.

A simple method is to vary the wrap angle in the last elastically/plastically acting roller and thereby to shift the residual curvature profile shown in FIG. 3 upward or downward. Also conceivable here is a closed feedback control system. Furthermore, it can make sense to measure the cross bow after each elastic/plastic bending in order to optimize the setting of all wrap angles and to be able to compare it with the calculation model. For this, measuring devices can be provided that are connected to a controller working with or without feedback. This is not illustrated in the figures. With a finite element model, for example, the theoretically optimal cross bow downstream of each roller can be anticipated. Subsequently, the rollers can be set in such a manner that the measured cross bows match the calculated ones as well as possible. It can then be assumed that the actual straightening process deviates only minimally from the theoretically calculated one, and therefore also the straightening result is a good match.

As already explained, the position-controlled adjustment (transverse to the strip travel direction) is preferably carried out by finely adjustable screw jacks. Such a fine adjustment works usually with an adjustment speed of 2 to 3 mm/sec. For passing through a joint of two coils, for example a punch connection or a welding seam, it can optionally be useful to open the stretch-bending stand, that is, the rollers are moved apart. If the distances to be traveled in the course of this are long, this process with the finely adjustable screw jacks takes a relatively long time. For this reason, the deflection rollers 4 a provided upstream and the deflection rollers 4 b provided downstream are provided in the embodiment illustrated in FIG. 2. In this manner, the adjustment spacings are considerably shortened.

Alternatively or additionally, there is the possibility of providing, in addition the fine adjustment (via for example screw jacks), a quick adjusting mechanism. This possibility is suitable in particular for the embodiment according to FIG. 1, but can in principle also be provided for the embodiment according to FIG. 2. In this case, the straightening rollers are also provided with a quick adjusting mechanism, for example with hydraulic or pneumatic cylinder actuators. It can be advantageous to provide one common quick adjusting mechanism for a plurality of straightening rollers. For example, in the embodiment according to FIG. 1 it can be advantageous to mount the first straightening roller and the third straightening roller on a common top frame that can be adjusted with one or a plurality of cylinder actuators. Correspondingly, the second straightening roller and the fourth straightening roller can be provided on a bottom frame that likewise can be displaced with one or a plurality of cylinder actuators. The quick adjustment can be carried out, for example, between two dead stops for opening and closing the stretch-bend-leveling stand. These possibilities are not illustrated in the figures. 

1. A method for continuously stretch-bend-leveling metal strips, where a strip under tension below the elastic limit is bidirectionally bent in the plastic or elastic range about at least four straightening rollers and thereby undergoes plastic stretching, wherein the bending radii at all four straightening rollers are set individually and independently of each other.
 2. The method according to claim 1, wherein for setting the bending radii, all four straightening rollers are adjusted in a position-controlled manner.
 3. The method according to claim 1, wherein the strip is bent about more than four straightening rollers, wherein the bending radii, at least at the last four straightening rollers, are set individually and independently of each other by individually adjusting preferably at least the last four straightening rollers in a position-controlled manner.
 4. The method according to claim 1, wherein the positions of the straightening rollers are set with an accuracy, for example control accuracy, which corresponds to an accuracy with regard to the wrap angles of the strip about the straightening rollers of ±0.05° or less, preferably ±0.02° or less.
 5. The method according to claim 1, wherein the wrap angles of the strip about one or a plurality of straightening rollers, preferably about all four straightening rollers are 0.5° to 60°.
 6. The method according to claim 1, wherein the stretch ratio is 0.1% to 1.5%.
 7. The method according to claim 1, wherein the tension in the strip is 10% to 90% of the elastic limit of the strip or the strip material.
 8. The method according to claim 1, wherein the spacing between at least two straightening rollers provided one behind the other with regard to the travel direction of the strip is at least 15% of the maximum strip width.
 9. The method according to claim 1, wherein the spacing between two straightening rollers provided one behind the other is at least 150 mm.
 10. The method according to claim 8, wherein the spacing between two straightening rollers provided one behind the other is provided at all four straightening rollers.
 11. The method for continuously stretch-bend-leveling metal strips, wherein a strip under tension below the elastic limit is bidirectionally bent in the plastic or elastic range about at least four straightening rollers and thereby undergoes plastic stretching according to claim 1, wherein the spacing in the travel direction of the strip between at least two straightening rollers is set in a variable manner.
 12. The method according to claim 1, wherein the target values for the positions of the straightening rollers or the target values for the wrap angles of the strip about the straightening rollers are determined with a calculation model that takes into account at least stress in longitudinal and transverse directions and, as calculation parameters, processes the strip thickness, the modulus of elasticity, the Poisson's ratio, the cyclic strength behavior of the strip, the tension in the strip, the roller radii or the geometry of the strip track about the rollers, or the target values of the strip thickness or the yield strength.
 13. The method according to claim 12, wherein the target values are determined with the calculation model with the proviso that the residual longitudinal curvature and the residual transverse curvature becomes zero.
 14. The method according to claim 12, wherein the target values are determined with the calculation model with the proviso that the residual curvature becomes zero and the residual longitudinal curvature takes a defined value.
 15. The method according to claim 1, wherein downstream of one or a plurality of straightening rollers the cross bow of the strip over the strip width is measured.
 16. The method according to claim 15, wherein the feedback control of the position or wrap angle is carried out taking into account the obtained measured values for the cross bow.
 17. The method according to claim 16, wherein the feedback control is based on a calculation model that considers at least stress in the longitudinal and transverse directions and, as calculation parameters, processes the strip thickness, the modulus of elasticity, the Poisson's ratio, the cyclic strength behavior of the strip, the tension in the strip, the roller radii and the geometry of the strip track about the rollers, and a certain cross bow is associated with a residual longitudinal or transverse curvature.
 18. The method according to claim 1, wherein the calculation model or the calculation models is/are based on the finite element method by the expected maximum nonplanarity of the strip or the nonplanarity of the strip measured prior to straightening, the optimal stretch ratio or the elastically/plastically acting straightening rollers' optimal horizontal spacings, which result in the minimal residual nonplanarity, are calculated using the finite element methods.
 19. An apparatus for continuously stretch-bend-leveling metal strips according to a method according to claim 1, comprising at least four straightening rollers about which a strip under tension below the elastic limit is bidirectionally bent in the plastic or elastic/plastic range, and further comprising at least one controller working with or without feedback, wherein the bending radii at all four straightening rollers can be set individually and independently of each other.
 20. The apparatus according to claim 19, wherein all four straightening rollers are connected to a controller and can be adjusted in a position-controlled manner.
 21. The apparatus according to claim 19, wherein the diameters of one or a plurality of straightening rollers and preferably all four straightening rollers are 15 mm to 150 mm.
 22. The apparatus according to claim 19, wherein of the four straightening rollers at least the last two straightening rollers are provided directly one behind the other without interposing further rollers.
 23. The apparatus according to claim 22, wherein the four straightening rollers are provided directly one behind the other without interposition of further rollers.
 24. The apparatus for continuously stretch-bend-leveling metal strips according to a method according to claim 1, comprising a plurality of straightening rollers about which a strip under tension below an elastic limit is bidirectionally bent in the plastic or elastic/plastic range wherein the spacing between at least two straightening rollers can be variably set in the travel direction of the strip.
 25. The apparatus according to claim 24, wherein the position of at least one straightening roller, preferably of all straightening rollers along the strip travel direction can be set.
 26. The apparatus according to claim 19, wherein upstream or downstream of at least one straightening roller, an adjacent deflection roller is provided, the deflection roller or the deflection rollers preferably having three times the diameter of the straightening roller.
 27. The apparatus according to claim 26, wherein the upstream deflection roller or the downstream deflection roller has a concave or convex roller shape.
 28. The apparatus according to claim 26, wherein the upstream deflection roller or the downstream deflection roller is equipped with a roller bending mechanism in the horizontal or vertical direction.
 29. The apparatus according to claim 19, wherein upstream of the first straightening roller a shape-variable tensioning roller or deflection roller is provided that is wrapped by the metal strip for example by at least 120°.
 30. The apparatus according to claim 19, wherein the straightening rollers can be adjusted by actuators.
 31. The apparatus according to claim 19, wherein individual position-adjustable straightening rollers or each of a plurality of position-adjustable straightening rollers are commonly and additionally provided with a quick adjusting mechanism. 